Whale shark
|
? Whale
shark Conservation status:
Vulnerable
|
|||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
 Whale shark in the
Maldives
|
|||||||||||||||
| Scientific classification | |||||||||||||||
|
|||||||||||||||
| Rhincodon
typus ( Smith, 1828) |
|||||||||||||||
|
|
The whale shark (Rhincodon typus) is a distinctively-marked member of the subclass Elasmobranchii of the class Chondrichthyes. It is the largest shark and also the largest fish. (The smallest known fish species is Paedocypris.)
Naming
The species was first identified in 1828 off the coast of South Africa. The family Rhincodontidae was not finalized until 1984. The name "Whale shark" comes from the fish's large size.
Distribution and habitat
The whale shark inhabits the world's tropical and warm-temperate oceans. While thought to be primarily pelagic, seasonal feeding aggregations of the sharks occur at several coastal sites like Ningaloo Reef in Western Australia as well as Pemba and Zanzibar on the coast of East Africa. Its range is restricted to about ±30 ° latitude from these sites. The whale shark is solitary and rarely seen in groups. It is believed to be migratory, but experts remain unclear as to how far migrations can occur ( transoceanic migrations have been suggested).
Anatomy and appearance
The greatest size accurately recorded was 12 meters (39 ft) long, with unofficial accounts of 18 meters (59 ft). It is not to be confused with the Basking shark (Cetorhinus maximus), the second largest fish. A member of the order Orectolobiformes, it is a filter feeder. The shark has a capacious mouth which can be up to 1.5m (5 ft) wide and contain up to 300 rows of tiny teeth. As part of its feeding process, it also has five large pairs of gill arches. Two small eyes are located towards the front of the shark's wide, flat head. The body is mostly grey with a white belly; three prominent ridges run along each side of the animal and the skin is marked with a 'checkerboard' of pale yellow spots and stripes. These spots are unique with every whale shark. Because of the spots uniqueness they can be used to mark each animal and make an accurate population count. But so far the whale sharks have not been fully and accuratly counted. Its skin can be up to 10 cm thick, the thickest skin of any animal. The shark has two pairs each of dorsal fins and pectoral fins. A juvenile whale shark's tail has a greater top fin than lower fin while the adult tail becomes semi-lunate, or crescent-shaped. The shark's spiracles are just behind the eyes. The whale shark is not an efficient swimmer; the entire body is in motion while the animal swims. The result of this motion, one that is very unusual for sharks, is an average speed of around 5 km/h.
Diet
The whale shark feeds on phytoplankton, macro- algae, plankton, krill or nektonic life (small squid or vertebrates). The many rows of teeth play no role in feeding, instead, water is actively drawn into the mouth and is passed over gill rakers and then out through the gill arches. Any material caught in the rakers is swallowed. The shark can circulate water at a rate up to 1.7 L/s (3.5 US pint/s). The sharks, however, are active feeders and target concentrations of plankton or fish by olfactory cues rather than simply 'vacuuming' constantly.
Behavior
When it is explained that not all sharks are dangerous to humans, this species is used as the leading example. Nature documentaries frequently show footage of divers keeping pace with the giant fish, and the shark pays them no mind. The whale shark is often seen by divers while feeding; it is very popular to have whale shark dive/snorkel trips in places like Ningaloo reef in Western Australia. The shark is often seen by divers in Thailand, The Maldives, The Red Sea and at the Galapagos islands. Lucky divers have also come across whale sharks in the Seychelles and in Puerto Rico.
Reproduction
Like most sharks, the reproductive habits of the whale shark are obscure. Based on the study of a single egg recovered off the coast of Mexico in 1956, it was believed to be oviparous, but the capture of a pregnant female in 1995 containing 300 young whale sharks indicates that they are viviparous with ovoviviparous development. The eggs remain in the body and the females give birth to live 40 to 60 cm young. It is believed that they reach sexual maturity at around 30 years and the life span has been estimated to be between 60 and 150 years.
Importance to humans
The whale shark is targeted by artisanal and commercial fisheries in several areas where they seasonally aggregate. The population is unknown and the species is considered vulnerable by the IUCN. Whale sharks are known to frequent the waters off Donsol in the Sorsogon province of the Philippines. A whale shark is also featured as the main attraction of Osaka Aquarium Kaiyukan.
As of 2005, three whale sharks are studied in captivity at the Okinawa Churaumi Aquarium in Japan. Two whale sharks are held in the Georgia Aquarium, which opened in 2005, in Atlanta.